Atrial fibrillation (AF) is a leading cause of strokes. Of the 700,000 strokes that occur annually in the United States, 80,000 (11.4 percent) were complications of AF. Currently, it is suggested that 62 to 91 percent of atrial thrombi in patients suffering from AF originate in the atrial appendages, and 90 percent of embolic strokes are a result of clots formed in the left atrial appendage (LAA). Obliteration of the LAA is estimated to reduce the annual stroke rate by up to 75 percent, while preservation of the right atrial appendage has been reported to maintain plasma levels of atrial natriuretic peptide and the kidneys' ability to excrete sodium. A rapid placement clip has been proposed to allow isolation of the left appendage from the rest of the circulatory system and reduce the risk of atrial thrombi induced strokes, without the morbidity of appendage excision. As envisioned, this clip will be able to occlude and isolate the LAA not only during an open heart surgical procedure, but also in a minimally invasive procedure without the use of cardiopulmonary bypass. As part of this work, the design parameters for a left atrial appendage occluding (LAAO) clip suitable for use on canines will be identified through a series of mechanical tests, and its efficacy will be demonstrated using a ninety day in vivo canine model. When completed, the LAAO clip will provide immediate occlusion of the appendage, will induce adhesion of the endo-appendiceal surfaces for long term occlusion, will rely on tissue ingrowth to provide long term positional stability, and will produce minimal effects on the surrounding tissues. Hemodynamic testing will be used to verify the efficacy of appendage occlusion and histologic methods will be used to evaluate the cellular response in the appendage and surrounding tissues to the presence of the clip. Phase II will focus on adapting the canine design for use in humans.